In upstream pneumatic conveying lines transporting bulk solid material, especially pulverized material, over larger distances, particularly in so-called dense phase conveying, and/or against a considerable back-pressure at the outlet of the conveying line(s), the overpressure required in order to feed the bulk material into the line(s) may be significant.
In these circumstances, the inward transfer of the bulk material is commonly performed by means of hoppers designed as pressure vessels and usually called conveying hoppers, feed hoppers, blow tanks, etc. In the frequent cases where a continuous supply of bulk material to the downstream consumer is requested, at least two of those hoppers are provided, either in a serial arrangement or in a parallel arrangement.
In case of a serial arrangement, the first hopper is operated as a lock hopper, being cyclically filled from an upstream storage bin or similar, pressurized, emptied into the second hopper and finally depressurized or vented, whereas the second hopper is constantly maintained in overpressure conditions and continuously feeds the pressurized bulk material into the conveying line(s).
In case of a parallel arrangement, both hoppers are operated as lock hoppers in a “staggered parallel mode”, i.e. are both cyclically filled, pressurized, emptied and depressurized, and alternately feed the pressurized bulk material into the conveying line(s), in such a way as to have a continuous supply of pressurized bulk material into this/these conveying line(s).
A typical example of lock hoppers operated as inward transfer equipment may be found in so-called Pulverized Coal Injection (PCI) Plants, supplying pulverized coal to blast furnaces. In those plants, operating overpressure levels of the hoppers in the range of about 5 bar g up to 20 bar g are common. Operating pressure levels of up to 30 bar g and more may be required e.g. in equipment feeding pulverized coal into coal gasifiers.
As described above, lock hoppers are thus operated batchwise or discontinuously by alternative cycles of filling in bulk material while the lock hopper is depressurized, closing and pressurizing the hopper and opening the hopper's outlet to convey the bulk material into the pressurized conveying lines or, in the case of the above-mentioned serial arrangement into a second hopper constantly under pressure. Lock hoppers are thus very different from continuously operated so-called blow bottles, such as those described in U.S. Pat. No. 5,265,983. Indeed, such blow bottles being constantly operated under pressure require complex feeding devices often consisting of a cascade of pressure resistant feeding units with transitional zones under increasing pressure. Especially for systems operating at high pressures such blow bottles are either not usable at all or become too complex and unreliable.
Pressurizing of the bulk material inside the lock hopper is performed by injecting pressurized process gas into the bulk material. In case the bulk material is combustible, e.g. in case of pulverized coal, the process gas is usually inert (has a reduced content of oxygen) in order to prevent fire and explosion. Compressed nitrogen is commonly used in such cases. The amount of process gas needed to pressurize the bulk material inside the hopper is conditioned by the inner volume of the hopper, the overpressure level to be achieved, the filling level of the bulk material and the void fraction of the bulk material (ratio of void volume to total volume). Void fraction of bulk material can be large, 60% and more, so that a completely filled hopper may require an amount of pressurizing gas in the order of magnitude of an empty hopper.
Process gas for pressurizing each lock hopper is supplied through a pressurizing gas branch, connecting the process gas supply main of the inward transfer sub-plant to the lock hopper to be pressurized. In order to shorten the hopper cycle time and thus the required capacity and inner volume of this lock hopper, while avoiding peaks in process gas demand on supply main level, the process gas may be accumulated in a buffer vessel for pressurizing gas. The buffer vessel is continuously filled with pressurizing gas supplied from the supply main at a reduced flow rate, and then periodically, each time a lock hopper is to be pressurized, emptied at a large flow rate into the lock hopper. Depending on the pressure level of process gas supply and the operating pressure level in the lock hopper, it may make sense to install two buffer vessels instead of one, performing pressurizing of the lock hopper at a large flow rate in two stages, and only partly pressurize the lock hopper with pressurizing gas accumulated in the buffer vessel(s), while the complementary amount of pressurizing gas is directly supplied from the process gas supply main to the lock hopper.
An important aspect of such installations is that the initial difference in gas pressure between the process gas supply main or the buffer vessel and the lock hopper is generally so high that it would lead to initially very huge gas velocities leading to compacting the bulk material inside the lock hopper, thereby reducing its flowability and thus preventing the subsequent emptying of the hopper. So, although the time needed to pressurize the lock hopper should be kept as short as possible in order not to be a limiting factor in the downstream process in common plants, the initial flow rates in the pressurizing gas branches are generally limited.
A non-expensive and thus commonly used way to limit pressurizing gas flow rates is to fit the related pipeline with a Laval tuyere. While only producing a limit pressure loss, a Laval tuyere limits the gas mass flow rate to values strictly proportional to the gas absolute pressure upstream the tuyere. Which means that the pressurizing gas flow rate supplied from the process gas supply main through the branch into the lock hopper is constant, provided the gas pressure level in this main is constant, and that the pressurizing gas flow supplied from the buffer vessel is decreasing over time as the pressure level in that vessel is decreasing.
The main disadvantages of existing/conventional installations remain the too long duration needed for fully pressurizing bulk material in the lock hopper, the dimensioning of certain parts of the equipment to cope with initial pressurizing conditions and/or the noise nuisance during the process.